Conventionally, in order to produce a three-dimensional sound field, there have been proposed many surround sound speaker systems of a type in which a plurality of speakers are provided around a listener. One of such systems is described in Unexamined Japanese Patent Publication No. H11-4500. FIG. 10 is a block diagram of a conventional surround sound speaker system, and a position and a facing direction of a viewer/listener are also shown in FIG. 10. FIG. 10 illustrates a system in combination with video images.
To television set 201 that presents a video image, interconnection module 203 is connected. With this, a sound signal of television set 201 is outputted to interconnection module 203. To interconnection module 203, subwoofer bass speaker 205, as well as front-center speaker 207, left-satellite speaker 209, right-satellite speaker 211, and rear-ambience speaker 213 that are respectively positioned front-side, left-side, right-side, and rear-side of viewer/listener 215 are connected. Therefore, interconnection module 203 has a function of generating various signals including, in addition to right and left sound signals, a sum signal of the right and left sound signals, and a difference signal between the right and left sound signals, and of outputting these signals to the five speakers.
Sound pressure P when a sound signal is emitted from each speaker in such a configuration is shown in FIG. 11A and FIG. 11B. Here, FIG. 11A is a characteristic diagram of sound pressure P of audible sound at distance d from television set 201 and front-center speaker 207 to a position of viewer/listener 215 in a front-back direction represented by line Y-Y. FIG. 11B is a characteristic diagram of sound pressure P along an interval between left-satellite speaker 209 and right-satellite speaker 211 through the position of viewer/listener 215, that is, distance w in a right-left direction represented by line X-X. In both of the figures, sound pressure P emitted from each speaker is shown normalized such that its maximum value takes 1.
Typically, a conventional speaker is called as a dynamic speaker, and configured such that a permanent magnet is provided within a yoke constituted by a magnetic body such as iron, and a magnetic field is produced by converging magnetic flux of the permanent magnet around a voice coil based on a configuration of the yoke. At this time, supplying an alternating current to the voice coil causes the voice coil to vibrate receiving a Lorentz force from the magnetic field produced by the yoke in a vertical direction (thickness direction of the yoke), and thus causes air to vibrate via a diaphragm, also called as a corn, connected to this voice coil, and whereby sound is produced.
Therefore, the sound pressure produced from the normal speaker is maximized near the speaker and decays in the process of the sound propagating through the air due to absorption and diffusion to the air, and thus the sound pressure decreases as the distance from the speaker increases.
Further, as an angle of an aperture of the diaphragm with respect to a sound axis lying along a direction in which sound waves from the speaker travel is large, a directional angle of the sound wave emitted from the normal speaker is often large.
As the speakers that constitute this surround system are normal speakers, sound pressure P of front-center speaker 207 is maximized at a position of front-center speaker 207, and decreases as distance d increases, as illustrated in FIG. 11A. Further, sound pressure P of rear-ambience speaker 213 is also maximized at a position of rear-ambience speaker 213, but decreases as distance d decreases. Specifically, sound pressure characteristics of front-center speaker 207 and rear-ambience speaker 213 at distance d are opposite from each other with respect to the front-back direction of viewer/listener 215. Therefore, as shown by a heavy line in FIG. 11A, superimposed sound pressure P from front-center speaker 207 and rear-ambience speaker 213 is maximized at the position of viewer/listener 215. Here, superimposed sound pressure P emitted from front-center speaker 207 and rear-ambience speaker 213 is also shown normalized such that its maximum value takes 1.
Similarly, as illustrated in FIG. 11B, sound pressure P of left-satellite speaker 209 is maximized at a position of left-satellite speaker 209, and decreases toward the right side within distance w. Further, sound pressure P of right-satellite speaker 211 is maximized at a position of right-satellite speaker 211, and decreases toward the left side within distance w. Thus, sound pressure P of left-satellite speaker 209 and sound pressure P of right-satellite speaker 211 show characteristics opposite from each other with respect to the right-left direction of viewer/listener 215. Therefore, as shown by a heavy line in FIG. 11B, superimposed sound pressure P from left-satellite speaker 209 and right-satellite speaker 211 is maximized at the position of viewer/listener 215.
A combination of the sound pressure characteristics shown in FIG. 11A and FIG. 11B in the front-back direction and in the right-left direction with respect to viewer/listener 215 is as shown in FIG. 12. Sound pressure P is maximized at the position of viewer/listener 215 at distance d in the front-back direction and within distance w in the right-left direction. With this, viewer/listener 215 is able to listen to the sound from front, rear, right, and left, and surrounded by the sound, and thus a three-dimensional effect can be produced.
According to the surround sound speaker system as illustrated in FIG. 10, while a three-dimensional effect can be produced for viewer/listener 215 with this system, it is necessary to provide a large number of speakers around viewer/listener 215, and therefore there is a problem that this system not only occupies a large area, but also makes wiring cumbersome.